U.S. patent number 7,072,540 [Application Number 10/421,284] was granted by the patent office on 2006-07-04 for method of assembling a multiplexer/demultiplexer apparatus to account for manufacturing variations in the thin-film optical filters.
This patent grant is currently assigned to Raytheon Company. Invention is credited to Stanislaw Szapiel, James R. Whitty.
United States Patent |
7,072,540 |
Szapiel , et al. |
July 4, 2006 |
Method of assembling a multiplexer/demultiplexer apparatus to
account for manufacturing variations in the thin-film optical
filters
Abstract
The thin-film optical filters and transmitted-light collimators
of a multistage optical multiplexer/demultiplexer apparatus are
assembled by locating the thin-film optical filters so that an
incident light beam is incident upon and reflected sequentially
from each thin-film optical filter to the next thin-film optical
filter, and orienting each thin-film optical filter to produce a
respective angle-of-incidence of the incident light beam on the
thin-film optical filter so that a desired transmitted light beam
is transmitted therethrough with a maximum intensity. The
transmitted-light collimators are positioned to receive the
respective transmitted light beams with minimal insertion loss. The
respective steps of positioning are performed independently of, but
after, the respective steps of orienting for each
multiplexer/demultiplexer stage.
Inventors: |
Szapiel; Stanislaw (Port
McNicoll, CA), Whitty; James R. (Midland,
CA) |
Assignee: |
Raytheon Company (Waltham,
MA)
|
Family
ID: |
36613825 |
Appl.
No.: |
10/421,284 |
Filed: |
April 22, 2003 |
Current U.S.
Class: |
385/24; 385/14;
385/34 |
Current CPC
Class: |
G02B
6/29365 (20130101); G02B 6/2938 (20130101); G02B
27/62 (20130101) |
Current International
Class: |
G02B
6/28 (20060101) |
Field of
Search: |
;385/24 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lee; John R.
Assistant Examiner: Johnston; Phillip A.
Attorney, Agent or Firm: Schubert; William C. Vick; Karl
A.
Claims
What is claimed is:
1. A method for assembling an optical multiplexer/demultiplexer
apparatus, comprising the steps of furnishing a component set
comprising a base an input collimator a first thin-film optical
tilter and a second thin-film optical filter, and a first
transmitted-light collimator and a second transmitted light
collimator; orienting the thin-film optical filters by the steps of
fixing said input collimator to the base first locating the first
thin-film optical filter so that a first incident light beam from
said input collimator is incident upon the first thin-film optical
filter, thereafter first orienting the first thin-film optical
filter to produce a first angle-of-incidence of the first incident
light beam on the first thin-film optical filter so that a desired
first-transmitted light beam is transmitted therethrough with a
maximum intensity and a second incident light beam is reflected
from the first thin-film optical filter, thereafter second locating
the second thin-film optical filter so that the second incident
light beam is incident upon the second thin-film optical tilter,
and thereafter second orienting the second thin-film optical filter
to produce a second angle-of-incidence of the second incident light
beam on the second thin-film optical filter so that a desired
second-transmitted light beam is transmitted therethrough with a
maximum intensity and a third incident light beam is reflected from
the second thin-film optical filter; and positioning the
transmitted-light collimators by the steps of first positioning the
first transmitted-light collimator to receive the first-transmitted
light beam with minimal insertion loss, wherein the step of first
positioning is performed independently of, but after, the step of
first orienting, and second positioning the second
transmitted-light collimator to receive the second-transmitted
light beam with minimal insertion loss, wherein the step of second
positioning is performed independently of, but after, the step of
second orienting.
2. The method of claim 1, wherein the method includes the
additional steps of fixing the first thin-film optical filter in
place, performed after the step of first orienting, fixing the
first transmitted-light collimator in place, performed after the
step of first positioning, fixing the second thin-film optical
filter in place, performed after the step of second orienting, and
fixing the second transmitted-light collimator in place, performed
after the step of second positioning.
3. The method of claim 2, wherein the steps of fixing include the
step of fixing the first thin-film optical filter, the first
transmitted-light collimator, the second thin-film optical filter,
and the second transmitted-light collimator to the support
base.
4. The method of claim 1, wherein the method includes an additional
step of directing the first incident light beam through the input
collimator.
5. The method of claim 1, wherein the step of first positioning is
performed after the step of first orienting but before the step of
second orienting.
6. The method of claim 1, wherein the step of first positioning is
performed after the step of first orienting and after the step of
second orienting.
7. The method of claim 1, wherein the step of furnishing the
component set includes the step of furnishing the component set
additionally comprising a third thin-film optical filter, and a
third transmitted-light collimator, and orienting the third
thin-film optical filter by the steps of third locating the third
thin-film optical filter so that the third incident light beam is
incident upon the third thin-film optical filter, and thereafter
third orienting the third thin-film optical filter to produce a
third angle-of-incidence of the third incident light beam on the
third thin-film optical filter so that a desired third-transmitted
light beam is transmitted therethrough with a maximum intensity and
a fourth incident light beam is reflected from the third thin-film
optical filter, thereafter positioning the third transmitted-light
collimator by the steps of third positioning the third
transmitted-light collimator to receive the third-transmitted light
beam with minimal insertion loss, wherein the step of third
positioning is performed independently of, but after, the step of
third orienting.
8. The method of claim 7, wherein the step of aligning the third
multiplexer/demultiplexer stage includes additional steps of fixing
the third thin-film optical filter in place, performed after the
step of third orienting, and fixing the third transmitted-light
collimator in place, performed after the step of third
positioning.
9. A method for assembling an optical multiplexer/demultiplexer
apparatus, comprising the steps of furnishing a component set
comprising a base, an input collimator and at least two
multiplexer/demultiplexer stages whose components are to be
oriented and positioned in sequence, wherein each
multiplexer/demultiplexer stage includes a respective thin-tilm
optical filter and a respective transmitted-light collimator;
orienting the thin-film optical filters by the steps of fixing the
input collimator to the base respectively locating each thin-film
optical filter so that an incident light beam from the input
collimator is incident upon and reflected sequentially from each
thin-film optical filter to the next thin-film optical filter,
thereafter respectively orienting each thin-film optical tilter to
produce a respective angle-of-incidence of the incident light beam
on the thin-film optical filter so that a desired transmitted light
beam is transmitted therethrough with a maximum intensity; and
positioning the transmitted-light collimators by the steps of
respectively positioning each respective transmitted-light
collimator to receive the respective transmitted light beam with
minimal insertion loss, wherein the respective step of positioning
is performed independently of, but after, the respective step of
orienting for each multiplexer/demultiplexer stage.
10. The method of claim 9, wherein the method includes the
additional steps of respectively fixing the thin-film optical
filters in place, performed after the step of respectively
orienting, and respectively fixing the transmitted-light
collimators in place, performed after the step of respectively
orienting.
11. The method of claim 10, wherein the steps of respectively
fixing include the step of fixing the thin-film optical filters and
the transmitted-light collimators to the support base.
12. The method of claim 9, wherein the method includes an
additional step of directing a first incident light beam through
the input collimator to a first multiplexer/demultiplexer
stage.
13. The method of claim 9, wherein the steps of respectively
locating and respectively orienting for each
multiplexer/demultiplexer stage are completed prior to performing
the steps of respectively orienting and respectively positioning
for the next sequential multiplexer/demultiplexer stage.
14. A method for assembling an optical multiplexer/demultiplexer
apparatus, comprising the steps of furnishing a component set
comprising a base, an input collimator and at least two
multiplexer/demultiplexer stages whose components are to be
oriented and positioned in sequence, wherein each
multiplexer/demultiplexer stage includes a thin-film optical filter
and a transmitted-light collimator, and sequentially aligning the
multiplexer/demultiplexer stages, wherein the step of sequentially
aligning includes the step of fixing the input collimator to the
base, the step of aligning the thin-film optical filter and the
transmitted-light collimator of each multiplexer/demultiplexer
stage to achieve a maximum intensity of a transmitted light beam of
a desired wavelength as received by the transmitted-light
collimator, wherein the orientation of the thin-tilm optical filter
and the position of the optical collimator are varied responsive to
a property of the thin-film optical filter.
15. The method of claim 14, wherein the method includes the
additional steps of respectively fixing the thin-film optical
filters in place, performed after the step of respectively
orienting, and respectively fixing the transmitted-light
collimators in place, performed after the step of respectively
orienting.
16. The method of claim 15, wherein the steps of respectively
fixing include the step of fixing the thin-film optical filters and
the transmitted-light collimators to the support base.
17. The method of claim 14, wherein the method includes an
additional step of directing a first incident light beam through
the input collimator to a first multiplexer/demultiplexer stage.
Description
This invention relates to optical apparatus, and more particularly,
to the assembly of a multiplexer/demultiplexer apparatus that
utilizes thin-film optical filters and optical collimators.
BACKGROUND OF THE INVENTION
Optical communications systems encode information onto a light beam
at a transmitting location, transmit the light beam through free
space or a medium such as an optical fiber, and then decode the
information from the light beam at a receiving location. A great
deal of information may be encoded onto the light beam due to its
high frequency. Additional information may be transmitted by
encoding it onto a second light beam having a slightly different
wavelength than the first light beam, mixing the two light beams
together at the transmitting location (or several different
transmitting locations), transmitting the mixed light beam,
separating the two light beams at the receiving location (or
several different receiving locations), and then decoding the two
sets of information from the two light beams. The amount of
information that may be transmitted is increased yet further by
using additional light beams in a similar manner, with all of the
light beams at slightly different wavelengths.
To implement such an optical communications system, the two or more
light beams having slightly different wavelengths must be mixed
together (i.e., combined into a single beam for transmission), a
process termed wavelength-division multiplexing, and later
separated apart from the single transmitted beam, a process termed
wavelength-division demultiplexing. The mixing and separating
operations are reciprocal, so that the same type of apparatus may
be used in a reciprocal manner, to perform both operations. The
apparatus used to perform the multiplexing and demultiplexing is
termed a multiplexer/demultiplexer apparatus, which may be
shortened to a "mux/demux apparatus."
One well-established mux/demux apparatus accomplishes the mixing
and separation with thin-film optical filters. When a light beam is
incident upon the thin-film optical filter at a precisely defined
angle of incidence, the thin-film optical filter passes light
within a very narrow spectral pass band characteristic of the
thin-film optical filter, and reflects all other wavelengths of
light. This property is used to mix light beams together or to
separate them, as required in the mux/demux apparatus. Many
variations of mux/demux apparatus use the thin-film optical filters
in the light communications systems.
However, the cost of such systems is substantially greater than
desired, due to the low manufacturing yield of suitable thin-film
optical filters. The thin-film optical filters are made by
depositing a sequence of typically over one hundred, and sometimes
as many as several hundred, very thin layers onto a substrate in a
pattern and thickness that, taken together, produces the desired
pass band characteristic. In a typical manufacturing operation, the
sequence of layers is deposited onto a large substrate, which is
then diced to produce a number of the individual thin-film optical
filters. It is often found that, with manufacturing variations in
this complex processing, only a small fraction of the final
thin-film optical filters satisfy the specifications that must be
met for the mux/demux apparatus to be functional as designed, when
the mux/demux apparatus is assembled using conventional
techniques.
There is therefore a need for an improved approach to mux/demux
apparatus and assembly approach that results in a satisfactory
sequence of mixing or separating the light beams, at a lower cost
than presently possible. The present invention fulfills this need,
and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a method for assembling an optical
multiplexer/demultiplexer (mux/demux) apparatus that utilizes
thin-film optical filters and light collimators. The method
utilizes a stepwise assembly approach that permits the use of
thin-film optical filters that are not perfectly formed according
to the design values. That is, thin-film optical filters may be
used even though these filters are somewhat off-specification due
to manufacturing variations. With conventional assembly approaches,
these off-specification thin-film optical filters would be
unusable, at least for the mux/demux apparatus. The present method
thereby improves the effective yield of usable thin-film optical
filters in the overall manufacturing process leading from the
fabrication of the thin-film optical filters to the final mux/demux
apparatus.
In accordance with the invention, a method for assembling an
optical multiplexer/demultiplexer apparatus comprises the steps of
furnishing a component set comprising a first thin-film optical
filter and a second thin-film optical filter, and a first
transmitted-light collimator and a second transmitted-light
collimator. The thin-film optical filters are oriented by the steps
of first locating the first thin-film optical filter so that a
first incident light beam is incident upon the first thin-film
optical filter, thereafter first orienting the first thin-film
optical filter to produce a first angle-of-incidence of the first
incident light beam on the first thin-film optical filter so that a
desired first-transmitted light beam is transmitted therethrough
with a maximum intensity and a second incident light beam is
reflected from the first thin-film optical filter, thereafter
second locating the second thin-film optical filter so that the
second incident light beam is incident upon the second thin-film
optical filter, and thereafter second orienting the second
thin-film optical filter to produce a second angle-of-incidence of
the second incident light beam on the second thin-film optical
filter so that a desired second-transmitted light beam is
transmitted therethrough with a maximum intensity and a third
incident light beam is reflected from the second thin-film optical
filter. The transmitted-light collimators are positioned by the
steps of first positioning the first transmitted-light collimator
to receive the first-transmitted light beam with minimal insertion
loss, wherein the step of first positioning is performed
independently of, but after, the step of first orienting, and
second positioning the second transmitted-light collimator to
receive the second-transmitted light beam with minimal insertion
loss, wherein the step of second positioning is performed
independently of, but after, the step of second orienting.
This approach is operable where the apparatus is subsequently used
as a multiplexer, where light beams are mixed together, or where
the apparatus is used as a demultiplexer, where light beams are
separated from the previously mixed light beam.
The method preferably includes the additional steps of fixing the
thin-film optical filters and transmitted-light collimators in
place, performed after they are located and oriented. In a typical
case, a support base is furnished, either in a form of a flat plate
or a more complex structure. The thin-film optical filters and
transmitted-light collimators are fixed to the support base by any
operable approach.
In most instances, the component set includes an input collimator
that is fixed in place, a first transmitted-light collimator and a
second transmitted-light collimator. The first incident light beam
is directed through the input collimator. The first incident light
beam serves as an alignment standard to which the other elements of
the component set are aligned. The input collimator is therefore
initially fixed in space and remains fixed for the remainder of the
assembly and alignment, preferably by permanently fixing it to the
support base, to fix the orientation of the first incident light
beam. After the first thin-film optical filter is oriented, the
first transmitted-light collimator is positioned to receive the
first-transmitted light beam with minimal insertion loss. After the
second thin-film optical filter is oriented, the second
transmitted-light collimator is positioned to receive the
second-transmitted light beam with minimal insertion loss. After
the thin-film optical filters and light collimators are positioned,
they are fixed in place.
There may be, and typically are, additional
multiplexer/demultiplexer stages in the form of additional pairs of
thin-film optical filters and associated transmitted-light
collimators. They are located, oriented, positioned, and fixed in a
similar sequential manner. Thus, for example, a third
multiplexer/demultiplexer stage includes a third thin-film optical
filter, and a third transmitted-light collimator. The third
thin-film optical filter is oriented by the steps of third locating
the third thin-film optical filter so that the third incident light
beam is incident upon the third thin-film optical filter, and
thereafter third orienting the third thin-film optical filter to
produce a third angle-of-incidence of the third incident light beam
on the third thin-film optical filter so that a desired
third-transmitted light beam is transmitted therethrough with a
maximum intensity and a fourth incident light beam is reflected
from the third thin-film optical filter. The third
transmitted-light collimator is thereafter positioned by the steps
of third positioning the third transmitted-light collimator to
receive the third-transmitted light beam with minimal insertion
loss, wherein the step of third positioning is performed
independently of, but after, the step of third orienting. The third
thin-film optical filter and the third transmitted-light collimator
are fixed in place.
As noted, there may be additional thin-film optical filters and
associated transmitted-light collimators arranged as respective
multiplexer/demultiplexer stages, e.g., fourth, fifth, sixth, etc.
multiplexer/demultiplexer stages. They are sequentially positioned
and fixed in the manner described above, with the positioning of
each multiplexer/demultiplexer stage being responsive to the
positioning of the preceding stage(s).
Many multistage multiplexer/demultiplexer designs using thin-film
optical filters are known in the art, and are manufactured as
designed. The designs are specified in terms of the positions and
orientations of the thin-film optical filters under the assumption
that each of the thin-film optical filters is almost perfectly
manufactured to the required tight specifications. For example, it
is customary to specify for 100 GHz Dense Wavelength Division
Multiplexing (DWDM) that the filters shall work at incidence angles
of 1.8+/-0.1 degrees, and that the center wavelength offset error
at 1.8 degrees shall not exceed +/-0.05 nm. Practical experience
shows, however, that a substantial fraction of the actually
manufactured thin-film optical filters are close to the design
specifications, but suffer from slight deviations from the
specifications in respect to the angle of incidence required to
achieve the transmission of the pass band wavelength. The actually
manufactured thin-film optical filters also vary in their geometry,
with a slight inclination between the front and back faces of the
thin-film optical filters, termed a "wedge error". In manufacturing
practice, the wedge error varies between the individual ones of the
thin-film optical filters. All of these slightly deviating
thin-film optical filters would not be usable in the conventional
assembly approaches, resulting in a low manufacturing yield of
suitable thin-film optical filters. The conventional assembly
approaches specify the required locations and orientations of the
thin-film optical filters in the final optical mux-demux apparatus.
The thin-film optical filters that do not function at these
specified locations and orientations cannot be used. Thus, the
conventional assembly approaches do not address this problem of
nonuniformity in the as-manufactured thin-film optical filters, but
instead essentially require that almost-perfect filters be used. In
current production practice, the acceptable thin-film optical
filters are sorted out and used in the multiplexer/demultiplexer
assemblies, while the remainder are discarded or used in
less-critical applications. The result is a low manufacturing yield
of thin-film optical filters suitable for multiplexer/demultiplexer
apparatus, and accordingly a high cost of each operable
multiplexer/demultiplexer apparatus.
In the present approach, by contrast, the assembly procedure takes
each thin-film optical filter for what it is, whether perfect or
imperfect, and then locates and orients it according to its
angle-of-incidence pass band characteristics. The associated
transmitted-light collimator is positioned as required to achieve a
low insertion loss. The beam reflected from the thin-film optical
filter is used as the incident light beam to the next thin-film
optical filter, which is similarly positioned and oriented as
required to its incident light beam. The result is an optical
multiplexer/demultiplexer apparatus that uses a much higher
fraction of the available operable thin-film optical filters,
substantially increasing the manufacturing yield and reducing the
costs of the multiplexer/demultiplexer apparatus.
This approach of positioning the transmitted-light collimator
independently of the locating and orienting of its associated
thin-film optical filter results in slight variations in the
geometry of the multiplexer/demultiplexer apparatus assembly.
Consequently, the geometry of each assembly may be, and usually is,
slightly different from the geometry of every other assembly,
although the overall layouts are similar. However, the slight
irregularity allows the use of thin-film optical filters that would
not be usable if the assembly were required to adhere to a rigid,
precisely regular geometry for every multiplexer/demultiplexer
apparatus. The variations in geometry do not typically pose an
obstacle for the multiplexer/demultiplexer apparatus, because the
light inputs and outputs are all via flexible optical fibers
connected to the collimators.
The present approach thus utilizes a sequential aligning approach
proceeding from the first multiplexer/demultiplexer stage to the
last multiplexer/demultiplexer stage. However, this approach
permits the production of subassemblies. For example, the
first-third multiplexer/demultiplexer stages may be prepared as a
first subassembly in the manner described, and the fourth-sixth
multiplexer/demultiplexer stages may be prepared as a second
subassembly in the manner described. Then the input light beam of
the second subassembly is aligned to the output light beam of the
first subassembly. This subassembly approach is within the scope of
the present invention.
Other features and advantages of the present invention will be
apparent from the following more detailed description of the
preferred embodiment, taken in conjunction with the accompanying
drawings, which illustrate, by way of example, the principles of
the invention. The scope of the invention is not, however, limited
to this preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a thin-film optical filter;
FIG. 2 are schematic graphs of transmission as a function of angle
of incidence at a specific angle of incidence;
FIG. 3 is a block flow diagram of a method for practicing the
approach of the invention; and
FIG. 4 is a schematic view of an optical multiplexer/demultiplexer
apparatus.
DETAILED DESCRIPTION OF THE INVENTION
The present approach is prompted by the characteristics of
thin-film optical filters, which are illustrated in FIGS. 1 2. A
thin-film optical filter 20 is formed by depositing a number of
optical layers 22 on a transparent substrate 24. In a typical case,
there are over 100 of the optical layers 22, and often as many as
200 300 optical layers 22. An incident light beam 26 is directed at
an angle of incidence .theta..sub.1 against the substrate 24. A
transmitted light beam 28 having a pass band about a center
wavelength .lamda..sub.C passes through the thin-film optical
filter 20 and is typically received by a transmitted-light
collimator 30. A reflected light beam 32 having all other
wavelengths of the incident light beam 26 is reflected from the
thin-film optical filter 20 at the angle of incidence
.theta..sub.1. Such thin-film optical filters 20 and their
manufacture are known in the art.
In such thin-film optical filters 20, the transmitted wavelength is
tuned as the angle of incident .theta..sub.1 changes. Let
.lamda..sub.O be the center wavelength of light transmitted by the
filter when .theta..sub.1=0 (i.e., the beam is perpendicular to the
filter). If the filter is then rotated so that
.theta..sub.1.noteq.0, the transmitted center wavelength
.lamda..sub.C tunes according to the equation:
.lamda..sub.C(.theta..sub.1)=.lamda..sub.O(1-G
sin.sup.2.theta..sub.1).sup.1/2, where G is a dimensionless
coefficient characteristic for a given stack of layers 22 that
compose the thin-film optical filter 20. Note that
.lamda..sub.C<.lamda..sub.O, and the sign of the angle
.theta..sub.1 does not matter. The present assembly method is based
on this tunability property of such thin-film filters.
When the thin-film optical filter 20 is manufactured, the
transmission of the center wavelength .lamda..sub.C and pass band
are experimentally measured to occur at a incident light angle
.theta..sub.M. Ideally, .theta..sub.M would be precisely equal to a
specified incident light angle .theta..sub.S that is specified as
the angle of incidence .theta..sub.1+/-.delta., where .delta. is a
very small fraction of .theta..sub.1 allowed for an error, in the
thin-film optical filter that is to be manufactured. If that were
always the case, the present invention would not be required. But
in practice it is found that .theta..sub.M often does not equal the
specified incident light angle .theta..sub.S, and typically
deviates from .theta..sub.S by some amount. This deviation may be
due to any of a number of factors, such as small variations in the
composition and/or thickness of one or more of the optical layers
22, or variations across the wafer that is first manufactured and
then diced to form the individual thin-film optical filters.
It has been the practice in prior approaches for the assembly of
optical multiplexer/demultiplexer apparatus to require that
.theta..sub.M must equal .theta..sub.S, and then to select for use
only those thin-film optical filters that meet this criterion. Many
otherwise-acceptable thin-film optical filters must therefore be
discarded or diverted to other, less demanding uses.
The present approach is based in part upon the relaxation of the
requirement that .theta..sub.M must equal .theta..sub.S. Instead,
the assembly of the optical multiplexer/demultiplexer is custom
assembled to accommodate the individual value of .theta..sub.M for
each thin-film optical filter.
The assembly is also customized to precisely position the
transmitted-light collimators. Each thin-film optical filter is
ideally a perfectly planar device, with the front surface perfectly
parallel to the back surface. In practice, however, each thin-film
optical filter as actually manufactured is slightly wedge-shaped,
with the front surface and back surface slightly angularly oriented
to each other by an angle that varies from filter to filter. The
wedge error thus varies from thin-film optical filter to thin-film
optical filter. This wedge variability results in a slight angular
variation, from filter to filter, in the angle at which the
transmitted light leaves the thin-film optical filter. The wedge
variability error results in an increase, sometimes a severe
increase, in the insertion loss for the light coupled to the
respective transmitted-light collimators, relative to a
fixed-position input collimator.
In conventional practice that requires an identical geometry for
every multiplexer/demultiplexer apparatus, a thin-film optical
filter must be discarded if its wedge angle is too great. In the
present approach, in which the geometry is allowed to vary slightly
from apparatus to apparatus, after the thin-film optical filter is
oriented for maximum intensity of the desired transmitted
wavelength, the transmitted-light optical collimator is positioned
as necessary for minimum insertion loss.
By this approach of relaxing the requirement of regularity of
geometry of the assembly and custom locating and positioning both
the thin-film optical filter and the transmitted-light collimator
for each individual stage of the multiplexer/demultiplexer
apparatus in sequence, optimum performance is achieved even though
there is a production variation in the performance and/or geometry
of the thin-film optical filters. Production yields increase
because thin-film optical filters that were unusable in
conventional multiplexer/demultiplexer apparatus assemblies can be
used in the present approach.
FIG. 3 depicts a preferred approach for practicing the invention,
and FIG. 4 depict an optical multiplexer/demultiplexer apparatus 80
that may be assembled by the present approach. The following
discussion presents the alignment process in terms of a
demultiplexer application of the multiplexer/demultiplexer
apparatus 80, but the identical apparatus is used for multiplexer
applications by passing the light beams in the directions opposite
to the light paths shown in FIG. 4. That is, the
multiplexer/demultiplexer apparatus 80, once aligned, may be used
either for demultiplexer or multiplexer applications.
A set of components is furnished, step 40. The component set
includes all of the necessary components of the optical
multiplexer/demultiplexer apparatus 80. In the present
illustration, the optical multiplexer/demultiplexer apparatus 80
includes a support base 82 to which the remaining components are
ultimately affixed, an input collimator 84, and three
multiplexer/demultiplexer stages 86, 88, and 90. There may be
additional multiplexer/demultiplexer stages as needed. The first
multiplexer/demultiplexer stage 86 includes a first thin-film
optical filter 92 and a first transmitted-light collimator 94. The
second multiplexer/demultiplexer stage 88 includes a second
thin-film optical filter 96 and a second transmitted-light
collimator 98. The third multiplexer/demultiplexer stage 90
includes a third thin-film optical filter 100 and a third
transmitted-light collimator 102.
The input collimator 84, which provides the first incident light
beam passes therethrough to provide the standard to which the other
components are aligned, is fixed to the support base 82 by any
operable approach, step 42. The support base 82 may be a flat
plate, or it may be a structure with optical mounts. All of the
fixing steps herein may be performed by any operable approach, such
as an ultraviolet-curing adhesive or laser welding. The component
may be fixed directly to the base, as with the adhesive, or mounted
to an intermediate support which is then affixed to the base, as in
laser welding.
The first multiplexer/demultiplexer stage 86 is aligned, step 44,
by locating the first thin-film optical filter 92, step 46, so that
a first incident light beam 104 from the incident collimator 84 is
incident upon the first thin-film optical filter 92. (In FIG. 3,
the term "thin-film optical filter" is abbreviated as TFOF.) As
used herein "locating" the thin-film optical filter means
displacing the position of the thin-film optical filter along any
or all of three orthogonal axes so that the light beam incident
upon the thin-film optical filter matches (falls within) the clear
aperture of the thin-film optical filter. Usually, only a portion
of the thin-film optical filter transmits the proper wavelength
with a low insertion loss, this portion being termed the clear
aperture or optically active area of the thin-film optical filter.
The thin-film optical filter is displaced in three axes until the
incident light beam falls within this clear aperture.
The first thin-film optical filter 92 is angularly oriented, step
48, to produce a first angle-of-incidence of the first incident
light beam 104 on the first thin-film optical filter 92. As used
herein "orienting" a thin-film optical filter means angularly
rotating the thin-film optical filter about any of three orthogonal
axes to achieve the required angle-of-incidence for the desired
wavelength of light. This first angle-of-incidence is selected so
that a desired first-transmitted light beam 106 (i.e., the desired
center wavelength for that particular multiplexer/demultiplexer
stage) is transmitted therethrough with a maximum intensity, and a
second incident light beam 108 is reflected from the first
thin-film optical filter 92. The first thin-film optical filter 92
is thereafter fixed to the support base 82, step 50, in the
location and orientation previously established in steps 46 and
48.
The first transmitted-light collimator 94 is thereafter positioned
to receive the first transmitted light beam 106 with minimal
insertion loss, step 52. As used herein, "positioning" a
transmitted-light collimator means to displace and/or rotate the
collimator so that the light beam incoming to the collimator is
along the optical axis of the collimator and focuses exactly on the
optical fiber tip to provide maximum coupling efficiency and
minimum insertion loss. The first transmitted light beam 106 is
thereby separated from the first incident light beam 104 by the
first thin-film optical filter 92, and received by the first
transmitted-light collimator 94. Thereafter, the first
transmitted-light collimator 94 is fixed to the support base 82,
step 54, in the position previously established in step 52.
Thereafter, the second multiplexer/demultiplexer stage 88 is
aligned, step 56, by locating the second thin-film optical filter
96, step 58, so that the second incident light beam 108, previously
reflected from the first thin-film optical filter 92, is incident
upon the second thin-film optical filter 96. The second thin-film
optical filter 96 is angularly oriented, step 60, to produce a
second angle-of-incidence of the second incident light beam 108 on
the second thin-film optical filter 96. This second
angle-of-incidence is selected so that a desired second
transmitted-light beam 110 (i.e., the desired center wavelength for
that particular multiplexer/demultiplexer stage) is transmitted
therethrough with a maximum intensity, and a third incident light
beam 112 is reflected from the second thin-film optical filter 96.
The second thin-film optical filter 96 is thereafter fixed to the
support base 82, step 62, in the location and orientation
previously established in steps 58 and 60. The second
transmitted-light collimator 98 is positioned to receive the second
transmitted-light beam 110 with minimal insertion loss, step 64.
The second transmitted light beam 110 is thereby separated from the
second incident light beam 108 by the second thin-film optical
filter 96, and received by the second transmitted-light collimator
98. Thereafter, the second transmitted-light collimator 98 is fixed
to the support base 82, step 66, in the position previously
established in step 64.
Thereafter, the third multiplexer/demultiplexer stage 90 is
aligned, step 68, by locating the third thin-film optical filter
100, step 70, so that the third incident light beam 112, previously
reflected from the second thin-film optical filter 96, is incident
upon the third thin-film optical filter 100. The third thin-film
optical filter 100 is angularly oriented, step 72, to produce a
third angle-of-incidence of the third incident light beam 112 on
the third thin-film optical filter 100. This third
angle-of-incidence is selected so that a desired third
transmitted-light beam 114 (i.e., the desired center wavelength for
that particular multiplexer/demultiplexer stage) is transmitted
therethrough with a maximum intensity, and a fourth incident light
beam 116 is reflected from the third thin-film optical filter 100.
The third thin-film optical filter 100 is thereafter fixed to the
support base 82, step 74, in the location and orientation
previously established in steps 70 and 72. The third
transmitted-light collimator 102 is positioned to receive the third
transmitted-light beam 114 with minimal insertion loss, step 76.
The third transmitted light beam 114 is thereby separated from the
third incident light beam 112 by the third thin-film optical filter
100, and received by the third transmitted-light collimator 102.
Thereafter, the third transmitted-light collimator 102 is fixed to
the support base 82, step 78, in the position previously
established in step 76.
If there are additional multiplexer/demultiplexer stages, they are
aligned sequentially in order following this same general approach.
("Sequential" refers to the order in which the light beam passing
through the apparatus is incident upon the mux-demux stages and
their respective thin-film optical filters. The first mux/demux
stage contains the thin-film optical filter first encountered by
the light beam, the second mux/demux stage contains the thin-film
optical filter second encountered by the light beam after passing
through the first mux/demux stage, and so on.) As a result, all of
the multiplexer/demultiplexer stages are aligned to the original
first incident light beam 104 and with optimized, tuned locations
and orientations of the thin-film optical filters and collimators.
By this approach, manufacturing variations in the components from
ideal values are accommodated.
In each case, the respective positioning step of the
transmitted-light optical collimator for any particular
multiplexer/demultiplexer stage is performed after the orienting
step of the thin-film optical filter. Within this constraint, there
are a number of acceptable alignment variations. For example, the
orienting steps 48, 60, and 72 for the thin-film optical filters
may all be performed prior to the positioning steps 50, 62, and 74
of the transmitted light optical collimators. In another example,
the orienting step and the positioning step for each
multiplexer/demultiplexer stage may be completed prior to
performing those same steps for the next multiplexer/demultiplexer
stage.
Although a particular embodiment of the invention has been
described in detail for purposes of illustration, various
modifications and enhancements may be made without departing from
the spirit and scope of the invention. Accordingly, the invention
is not to be limited except as by the appended claims.
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